If, the phone is already unlocked then the roaming charges do not affect you?

Yes they do, unless you take out the Telus SIM card and put an ATT or TMobile one. You can also add a north american plan, but for such a short period of time I would not do that.

You can turn your data off when roaming (this is available under: options>mobile network). That will save you data roaming charges and will allow you to have your BB on during your travels in the states.